This invention relates generally to laser beam projectors or telescopes, and, more particularly, to a wavefront sampling system which utilizes a diffraction grating on the primary mirror of the beam projector in order to insure high accuracy in the projected beam.
Laser beams have a number of remarkable properties. Because of their spatial coherence, they have an extremely small divergence and are therefore highly directional. A laser beam because it possesses space coherence, can be focused to form a spot whose diameter is of the order of one wavelength of the laser light itself. Enormous power densities are thus obtainable. Accordingly, system applications of lasers are useful for communication in space, on earth and undersea, as well as in survelliance and weapon systems.
In many laser systems it is desirable to concentrate the projected laser energy into a small area in the object plane. In order to accomplish this end, large optics in the projector system are required. To achieve the high performance theoretically possible with such large optics, other parts of the projection system must also perform at comparable levels of high accuracy. For example, at diameters of 4.3 meters with .lambda.=2.7.mu., the radius of the Airy disc is only 0.77 R. To deposit energy upon a given target area at 3/4 of the ideal rate, the boresight error can be only about 0.2 .mu.R even assuming a perfect projected beam. On the other hand, if we assume a perfect boresight, the projected wavefront error allowable to achieve the 3/4 maximum deposition rate is only .lambda./13 rms.
To assure this high accuracy in the projected beam, a wavefront sampling system should incorporate therein the following characteristics:
1. It should have a negligible insertion loss; PA1 2. It should provide a signal to the wavefront sensors which is sufficiently and uniformly attenuated; PA1 3. It should sample over the entire aperture; PA1 4. It should give a measurement of the projected wavefront phase accuracy after the wavefront has left the last optical surface; PA1 5. It should be independent of those factors which do not affect energy density on the target (that is, laser wavelength changes, etc.); PA1 6. It should measure net projected wavefront tilt (boresight) as well as relative wavefront inaccuracies; and PA1 7. The wavefront analysis method should be capable of sufficient accuracy and signal to noise for general application.
The wavefront sampling systems which have been used in the past utilized therein either beamsplitters, corner cubes, or target return signals. Unfortunately, these systems have serious drawbacks for large aperture high energy wavefront sampling systems. For example, the beamsplitter samples from a distorting surface and does not measure the final projected wavefront. The corner cubes waste energy, sample over small aperture areas and suffer from diffraction effects. The use of the target return from distant targets suffers from a lack of high frequency response due to the light transit time, difficulty in using extended targets and signal to noise problems caused by a low target return and competition from the projector radiation. It is therefore clearly evident that there exists a need for a reliable wavefront sampling system which eliminates the drawbacks of sampling systems now in use and set forth hereinabove.